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Wednesday, May 7, 2014

GPS/GNSS Backup eDLoran System Delivers 5-Meter Accuracy

As reported by GPS World: Durk van Willigen, René Kellenbach, and Cees Dekker of the Dutch
consulting firm Reelektronika, and Wim van Buuren of the Dutch Pilots’
Corporation authored the ENC presentation about enhanced differential
Loran (eDLoran), with results that greatly — and pleasantly — surprised many in
the audience. A full technical article by these authors, describing the
equipment, methodology, and test results of eDLoran, will appear in the
July issue of GPS World. The new Loran project arose from the need of harbor pilots
responsible for bringing large and super-large freight ships into dock.
These pilots require GNSS-level acuracies of 5 meters for such work,
and all parties concerned — pilots, captains, ship owners, harbor
management — need some form of robustness, that is, back-up for the GNSS
systems in case of jamming, unintentional interference, system failure,
or other disruption.As extensive research had established that 5-meter accuracy cannot be
met by the currently tested DLoran system, which cannot get better than
10-meter accuracy. Reelektronika developed a new differential Loran
system called enhanced differential Loran, or eDLoran. A full prototype
eDLoran system was built and extensively tested in the Europort
(Rotterdam) area. The tests achieved accuracies of 5 meters.For maritime applications, eLoran is considered as the most promising
backup for GNSS in case the use of satellite-based navigation signals
is denied. The Dutch Pilots’ Corporation askedReelektronika to
investigate whether differential Loran could meet the pilots’ 5-meter
accuracy requirement for a harbor navigation. This proved to be an
enormous challenge as preliminary tests showed that even 10 meters was
difficult to achieve with differential Loran (DLoran) as promoted by the
UK’s Trinity House/General Lighthouse Authority (see item below about
Harwich UK tests by GLA and ACCESS). The challenge had led to a thorough
investigation of all possible error sources of a complete differential
Loran system.Differential techniques developed and implemented for Loran are
comparable with differential GPS. Although the error sources of GPS and
Loran are quite different, the major common error source in both systems
is the lack of accurate propagation models.This led to a new research project to find a more accurate
differential Loran technique. All possible error sources have been
investigated again where possible, which resulted in some unexpected
results regarding accuracies and costs.Enhanced Differential Loran: eDLoranThe new concept of differential Loran had to fulfill two important
primary improvements. The first is a significant reduction in the
latency of the data in the data channel; the second is that a large
number of reference stations should be capable of receiving the data
channel, without saturating the data channel. The simple conclusion was
that Eurofix could not meet these two improvements. However, Eurofix is
still the prime GNSS backup candidate for distributing accurate UTC over
very large parts of Europe. Further, Eurofix has the capability to send
short messages that might be encrypted for secure communication
purposes which might then form a terrestrial backup, for example,
Galileo PRS.Instead of using the Eurofix channel, eDLoran uses the public mobile
GSM (Global System for Mobile) network to send the differential
corrections to users. eDLoran receivers therefore contain a simple modem
for connection to the GSM network. The eDLoran reference stations are
also connected to the Internet which may be implemented via a cabled
access or also via a GSM modem.Fortunately, today many GSM networks are
robust in respect of GPS outages.The eDLoran infrastructure is not connected with any eLoran
transmitter station and operates completely autonomously. An eDLoran
reference station is connected to a central eDLoran server by its
connection to the network.eDLoran ResultsBoth static and dynamic tests have been carried out. Here, only the
final result of the dynamic test is presented. For full details on both
sets of tests, see the upcoming full-length technical article in the
July issue of GPS World magazine.The results have been demonstrated to the harbor authorities in
real-time on the laptop of the pilots on which the GPS-RTK and the
eDLoran position were simultaneously shown. The logged GPS-RTK data is
plotted on a Google Earth map shown in the accompanying figure. The
track was widened to 10 meters as the accuracy requirements are 5 meters on either side of the track. The raw eLoran track is also shown, as
well as the final white eDLoran track.

The
red track is based on raw eLoran data without any corrections. The
transparent blue line is made by GPS-RTK and is widened to 10 metres
giving the required ± 5 meter limits of eDLoran. The white line is
output from the eDLoran receiver which stays within the borders of the
10-meter-wide transparent blue line.

Conclusions

The outcome of the research opens some new and quite surprising
possibilities for multiple applications. Only a few of the authors’
conclusions appear here:

eDLoran offers the best possible eLoran accuracy as it does not
suffer from swaying wire antennas, sub-optimal timing control of the
transmitter station and differential data latency.

There is no need to replace older Loran-C stations with eLoran
transmitters saving large amounts of money. The existing Loran stations
have a proven reliability track record. Further savings may be obtained
by containerising the transmitter and operating the stations unmanned.

As there is no data channel bandwidth limitation, multiple reference
stations can be installed which offers increased reliability and makes
the system more robust against terrorism and lightning damage.

A single or multiple eDLoran servers can be installed in a protected
area. There is hardly a practical limit in the number of differential
reference stations to serve.

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I have more than 25 years of experience in development, design, and mobile communications products and technology. I also enjoy skiing, hiking, scuba, tennis, reading, traveling, foreign languages, and painting. I'm an active member of the National Ski Patrol (NSP) and volunteer my time at either Loveland Ski resort, or Ski Cooper.